That's the promise from the Secretary of Agriculture Schafer and the topic of discussion on the various blogs here and here and here
Now in 1956 President Eisenhower vetoed a farm bill because it provided for high rigid price supports. (The report of the veto wasn't accessible, but the link has Sen. Knowland threatening the veto, which actually happened.) In 1973 Nixon threatened one, and Ford did veto in 1975 (an emergency increase in target prices and supports--he eventually acted administratively in 1976 after firing Earl Butz and seeing defeat looming in the elections). And way back in 1927 Silent Cal vetoed McNary-Haugen. The NY Times archive site is having problems this morning, but the results of the query "farm bill veto" is here.
The process is similar each time--a coalition in Congress (easier to assemble back in the 1920' s when we had more farmers) gets behind legislation to benefit farmers, they go a bridge too far, and the President slaps them down. Of course, often the President threatens a veto but doesn't carry through--I wonder if a political scientist has done a study of that. It's part of the pull and haul of democratic politics.